The bainitic transformation in steels has been extensive

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The bainitic transformation in steels has been extensive
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Issue 4
DOI: 10.2478/amm-2014-0277
E. JEZIERSKA∗ , J. DWORECKA∗ , K. ROŻNIATOWSKI∗
NANOBAINITIC STRUCTURE RECOGNITION AND CHARACTERIZATION USING TRANSMISSION ELECTRON MICROSCOPY
ROZPOZNAWANIE I CHARAKTERYZACJA STRUKTURY NANOBAINITYCZNEJ ZA POMOCĄ TRANSMISYJNEJ
MIKROSKOPII ELEKTRONOWEJ
Various transmission electron microscopy techniques were used for recognition of different kinds of bainitic structures in
100CrMnSi6-4 bearing steel. Upper and lower bainite are morphologically different, so it is possible to distinguish between
them without problem. For new nanobainitic structure, there is still controversy. In studied bearing steel the bainitic ferrite
surrounding the retained austenite ribbon has a high density of dislocations. Significant fragmentations of these phases occur,
bainitic ferrite is divided to subgrains and austenitic ribbons are curved due to stress accommodation.
Keywords: transmission electron microscopy, nanobainite, bearing steel
W celu rozpoznania i scharakteryzowania poszczególnych morfologii bainitycznych w stali łożyskowej 100CrMnSi6-4
zastosowano różne techniki transmisyjnej mikroskopii elektronowej. Rozpoznawanie bainitu górnego i dolnego nie nastręcza
problemu, ze względu na ich zróżnicowane morfologie. W przypadku bezwęglikowego nanobainitu, ciągle jeszcze wiele jest
kontrowersji. W badanej stali łożyskowej ferryt bainityczny otaczający wstęgi austenitu szczątkowego cechuje się dużą gęstością
dyslokacji. Obydwie fazy wykazują znaczną fragmentację; ferryt bainityczny podzielony jest na podziarna a wstążki austenitu
ulegają wygięciu w wyniku akomodacji naprężeń.
The bainitic transformation in steels has been extensively studied, however it is still controversial whether it proceeds by a diffusional [1-2] or shear (displacive) [3-4] or
diffusional-displacive mechanism [5-6]. There are in detail
many elements to the controversy surrounding the mechanism
of formation of bainite until now.
In recent years Bhadeshia, Caballero, Garcia-Mateo and
coworkers [7-12] developed new kind of bainitic structure
named Nanobain. Low-temperature bainitic microstructure
can be obtained in high-carbon Si-rich steels by isothermal transformation for a long time (1-3 weeks). This resulting carbide-free bainitic microstructure consists of plates of
bainitic ferrite, which are just 20-40 nm in thickness, dispersed in a residue of carbon enriched retained austenite. The
achieved combination of mechanical properties is excellent,
with strengths in the range 1.6-2.5 GPa with a hardness of
about 650-700 HV, and toughness of 30-40 MPa/m2 , depending on the transformation conditions [12]. It was very inspired
to many researches and a lot of work was done during last
decade to achieve nanostructured bainitic steel with such good
properties [13-15]. The improvement in toughness reached in
high silicon bainitic steels is attributed to the replacement
of brittle interlath cementite of the upper conventional bainite
structure by interlath films of softer retained austenite. Carbide
precipitation can be suppressed during isothermal holding by
adding the right amount of silicon as an alloying element. The
∗
microstructure is carbide-free, not only because Si retards the
precipitation of cementite from austenite due to its low solid
solubility in the cementite crystal structure, but also because
a substantial quantity of carbon is trapped at accommodation twins and dislocations in vicinity of the ferrite-austenite
interface [ 16]. However, despite the high Si content in the
nanostructured bainitic steels (sim1.5 wt.%), evidence of Fe3 C
carbide formation has recently been found using advanced microscopic techniques, including Atom Probe Tomography [17].
A higher volume fraction of clusters and carbides were formed
after the isothermal transformation at 200◦ C for 10 days than
after transformation at 350◦ C for 1 day [18].
So, the question is, if it really carbide-free microstructure? What is the difference between dense lamellar pearlite
and nanobainite?
1. Material and methodology of investigations
The chemical composition of the steel used in this investigation was Fe 0.93-1.05%C 0.45-0.75%Si 1-1.2%Mn
1.4-1.65%Cr (wt.%). This commercial steel is widely used
in industry for bearings, so it is expected that enhancing
properties by nanostructurization will extend applications of
this very promising material. Studies of phase transformations occurring in these steel have been performed by dilato-
WARSAW UNIVERSITY OF TECHNOLOGY, FACULTY OF MATERIALS SCIENCE AND ENGINEERING, 141 WOŁOSKA STR., 02-507 WARSZAWA, POLAND
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metric measurements with the use of the Bähr’s DIL805L
dilatometer, in order to accurately design suitable heat treatment procedures. Specimens were austenitized for 30 minutes
at 930◦ C and then isothermally transformed at 320◦ C, for
5 hours and slow cool-down in ambient temperature. The
isothermal holding time enabled the total completion of the
bainitic transformation. For comparison, incomplete transformation was performed (90%) and lower austenitizing temperature (850◦ C/260◦ C-lower bainite) or isothermal holding at
680◦ C (pearlitic microstructure).
TEM specimens were prepared from dilatometric
heat-treated 3 mm rods. The foils used for the TEM were
cut into 0.2 mm thick slices, mechanically thinned to 0.07
mm and then twin-jet electropolished to perforation using a
Struers-Tenupol equipment with a mixture of 5% perchloric
acid in glacial acetic acid. Microstructure observations were
carried out on a JEOL JEM 3010 transmission electron microscope (TEM) operated at 300kV.
ters/nanocarbides are invisible on selected area electron diffraction, the spots are very weak.
2. Results and discussion
Nanostructured bainite was observed after isothermal
treatment in 320◦ C (Fig. 1). A nanometric bainitic structure
observed in studied steel fulfilling the nano-crystallinity criteria: the plate width for both phases was well below 100nm
(bainitic ferrite: 30-50nm, retained austenite: 14-20nm). For
proper estimation of volume fraction of nanobainitic structure more than 60 images of microstructure was recorded
around the thin foil perforation in each sample. From systematic observations it was concluded, that nanobainitic structure
is dominant in all the areas, achieving more than 80% of
the volume. Low Mag mode was very useful in this experiment due to visibility of extended areas with neighboring
prior austenite grains. In that way not only local arrangement
of nanobainitic structure was observed but also the connections between the prior austenitic grains. Crystallographic relationship between ferrite and austenite was determined using
superimposed selected area diffraction patterns in proper orientation of both phases. The orientation relationship between
ferrite and austenite was very close to Nishiyama-Wassermann
in studied nanobainitic structure.
In the case of well developed nanobainitic structure carbides were not observed, or only sporadically. The amount of
carbon and chromium in this steel favor carbides precipitation
(for bearings it is appreciable). The amount of silicon in this
steel is not sufficient to successfully avoid carbides abundance.
These carbides are not completely dissolved during
austenitizing, because of chromium enrichment. It was observed, that in many cases, cementite carbides inhered from
austenite are semicoherent with surrounding matrix and
smoothly embedded without structural discontinuity (Fig. 2a).
So, for this steel the terminology “carbide-free bainite” is
through only for well developed nanostructured bainite. During prolonged holding secondary precipitation can also occur
(Fig. 2b). These carbides are hidden in the microstructure
because of very small size below 5nm. Short range diffusion to dislocation core is sufficient for decorating dislocations with fine carbides. Because of very small size, similar interplanar spacings and small volume fraction these clus-
Fig. 1. Transmission electron micrographs of nanobainitic microstructure obtained at 320◦ C in Low Mag (a) higher magnification (b) and (c) selected area electron diffraction from carbide-free
nanobainitic structure with Nishiyama-Wassermann orientation relationship
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(Fig. 2c). The largest bainitic ferrite plates and retained austenite films were near this range. Looking at these microstructures in low magnification TEM or using SEM the mistake
is possible. But looking carefully in TEM at higher magnification with proper alignment and contrast enhanced with
objective aperture, the differences are evident (Fig. 3). The
most dominant difference is due to significant density of dislocations for nanostructured bainite. Transformation dislocations associated with front of the transforming parent phase
to transformation product are characteristic for displacive and
for diffusional-displacive transformation. In the area of bainitic
ferrite the density of dislocations are higher (Fig. 3a). Plate of
bainitic ferrite is divided to subgrains with additional dislocations on the subboundaries. In the vicinity of retained austenite
to ferrite interface these dislocations are clearly visible.
Fig. 2. TEM microstructure of 100CrMnSi6-4 bearing steel after
isothermal quenching; (a) at 320◦ C for 6 h, (b) at 320◦ C for 4h, (c)
at 680◦ C for 20 minutes
The second intriguing question was about similarity of
nanobainitic structure to dense lamellar pearlite. The smallest
pearlite lamellae for our steel were in the range 120-160 nm
Fig. 3. Transmission electron micrographs of nanobainitic microstructure obtained at 320◦ C, (a) bright field, (b) dark field with
(200) austenite spot
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Pearlitic transformation is diffusional, so the area of ferrite between cementite lamella is clear, nearly without dislocations.
In our steel nanostructured bainite is a little different than we can find in the microstructure presented by
Bhadeshia, Caballero, Garcia-Mateo and coworkers [19]. Instead of interpenetrated retained austenite film and bainitic
ferrite plates we have opposite morphology: retained austenite
ribbon/serpentine inside bainitic ferrite matrix (Fig. 3). Additionally, wider ferrite plates are divided to subgrains and
retained austenite ribbon is strongly deformed, fragmented to
finer segments and twins. The curvature of retained austenite
ribbon is in wavy manner. This behavior is connected with
stress accommodation and can be explained with theory of
stress induced interaction developed by Khachaturyan [20].
Another difference in morphology it is the absence of blocky
austenite. The benefit of this is manifested in reduction of
shape deformation in final product after applied heat treatment.
The results of mechanical tests for 100CrMnSi6-4 steel
with a nanobainitic structure after industrial heat treatment
[21] are very promising.
3. Summary and conclusions
Various transmission electron microscopy techniques
were used for recognition of different kinds of bainitic structures in 100CrMnSi6-4 bearing steel after isothermal quenching. A nanometric bainitic structure was observed in studied steel, fulfilling the nano-crystallinity criteria: the plate
width for both phases was well below 100nm (bainitic ferrite:
30-50nm, retained austenite: 14-20nm).
The orientation relationship between ferrite and austenite is very close to Nishiyama-Wassermann in studied
nanobainitic structure. The ribbons of austenite and bainitic
ferrite appear as packets of smaller sub-units. In studied bearing steel the ferrite surrounding the austenite ribbon has a high
density of dislocations.
Significant fragmentation of both phases occur, bainitic
ferrite is divided to subgrains and austenitic ribbons/lamellae
are curved due to stress accommodation.
Acknowledgements
The results presented in this work have been obtained within the
project NANOSTAL (contract no. POIG 01.01.02-14-100/09). The
project is co-financed by the European Union from the European Regional Development Fund within Operational Programme Innovative
Economy 2007-2013.
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